Brief Description of the Prior Art
It is known to produce gravel-size ceramic clay aggregate which is relatively light in weight. Such aggregate can be used in cement and concrete matrices materials used in construction. Several procedures are known for making such lightweight aggregate. One of the most effective processes for making a gravel-size lightweight aggregate of sufficient strength to resist fracturing, and usable in building materials, entails the initial step of mixing a relatively fine particulate ceramic clay with water in a batch blender apparatus, such as a pug mill. The water to clay ratio is adjusted to assure that a proper consistency similar to that of modeling clay is attained. Such consistency assures that the mixture will be susceptible to extrusion and particle formation in the subsequent steps of the process. Consistency determination may be effected by manual sampling, or by means of expensive moisture analyzers which continuously or periodically monitor the mixture to determine the moisture content, and, from this, the consistency of the mixture.
The mixture is discharged from the mixing zone into the hopper of an extrusion apparatus. The extruder is then used to force the clay composition through a series of small holes or orifices to form a series of long spaghetti-like rods. An automated chopping device periodically severes these rods at the forming orifices so as to form a quantity of small, relatively short cylinders having a length/width ratio of from about 1.0:1.0 to about 1.5:1.0.
The cylindrical particles thus formed are fed into a rotary drum dryer where the particles are dehydrated and rolled around the periphery of the drum to shape them into rough spheroids. The still damp spheroidal particles are discharged from the rotary drum dryer into the inlet of a rotary drum or a refractory lined kiln. Here they are fired for a period of about thirty minutes while they continue to roll against the inside surface of the rotating rotary drum kiln to complete the spheroidizing action. The pellets thus produced are from a size of about three-sixteenths inch to about one-quarter inch diameter with a hard outer shell, and a hollow to crusty interior caused by the expansion of trapped air and gas inside the "green" or damp pellets during the heating cycle. These pellets, when substituted for gravel in a concrete casting, provide a structure which is as strong as that containing the gravel for which they are substituted, and they can be used in place of the gravel at concentrations of up to fifty weight percent of the concrete composition. The lightweight of these particles enables a significant weight savings to be realized which reduces the transportation and handling cost in the building of precast concrete structures. Moreover, the void air spaces inside the lightweight aggregate particles provide concrete walls with greatly improved thermal insulating qualities.
Efforts to reduce the diametric size of the pellets have not been successful. The extrusion of the ceramic clay on a production basis appears to be limited to the use of extrusion ports having a diametric size of about one-eighth inch. Where attempts have been made to extrude the clay through smaller ports, the result has been plugging of the ports during the formation of the regular shaped pellets.
Attempts to produce smaller sized aggregate (than the gravel-size successfully used previously in cement and concrete matrices) by crushing and screening the expanded gravel-sized pellets have also not been successful. Breaking up the individual gravel-sized expanded pellets reduces the overall volume of the individual particles and increases the density by minimizing the effects of the internal cavities formed in the gravel-sized pellets by gas expansion. Moreover, undertakings to achieve size reduction by crushing and screening entails the use of expensive machinery, along with the necessary addition of other adjuncts to the material-handling system, which makes this type of operation economically prohibitive. Further, crushing and screening operations result in randomly produced oversized particles, along with very fine dust which is generated in the crusher, with a concomitant loss of from ten to fifteen percent of the total potential product through the scrapping of material that is outside of acceptable particle-size range.